Thin film photovoltaic (PV) modules (also referred to as “solar panels” or “solar modules”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy (sunlight) to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap (1.1 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts energy more efficiently in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (e.g., cloudy) conditions as compared to other conventional materials.
Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
Vapor deposition systems for processing individual glass sheets in the formation of PV modules are known. For all such systems, a vacuum lock or seal configuration must be provided at the ingress and egress points for the glass substrate as it moves into and out of the deposition chamber. For example, it is known to use vacuum locks (also referred to as “load locks”) for indexing discrete glass sheets into and out of a deposition chamber, as described in U.S. Pat. No. 4,797,054. These vacuum locks essentially seal the glass sheet within the chamber during the static deposition process and are not suitable for continuous conveyance of discrete glass sheets through a deposition chamber.
U.S. Pat. No. 5,772,715 describes a deposition system wherein multiple vacuum chamber slit seals are provided at the ingress and egress locations. These seals are relatively complicated and involve establishing three separate vacuum stages drawn by separate vacuum pumps. Although the U.S. '715 patent describes that the seals are suitable for processing either discrete glass sheets or a continuous glass ribbon, they are mechanically quite complex and would add significantly to the manufacturing costs of PV modules.
CSS (Close Space Sublimation) is a known commercial vapor deposition process for production of CdTe modules. Reference is made, for example, to U.S. Pat. No. 6,444,043 and U.S. Pat. No. 6,423,565. While there are advantages to the CSS process, the system is inherently a batch process wherein the glass substrates are indexed into a vapor deposition chamber, held in the chamber for a finite period of time in which the film layer is formed, and subsequently indexed out of the chamber. The U.S. '565 patent cited above describes a system wherein air-to-vacuum-to-air (AVA) openings are provided for indexing the glass substrates into and out of a vacuum chamber, which includes a processing station for vapor deposition of a film on the glass substrates. The patent purports to provide a “novel sealing relationship” between the deposition station (a heated “block” having a pocket in which the source material is contained) and the glass substrate wherein a relatively tight tolerance is maintained between the glass and the top of the block on the order of 0.001 in. to 0.018 in. This configuration results in the glass substrate essentially acting as a shutter across the top of the block pocket. This shutter-type of seal configuration wherein the glass substrate essentially seals the deposition chamber is not suitable for a continuous deposition process wherein discrete glass sheets are continuously conveyed through a vacuum deposition chamber.
Accordingly, there exists an ongoing need in the industry for a seal configuration that is particularly suited for large scale and efficient production of PV modules, particularly CdTe based modules, by continuous conveyance of a plurality of discrete substrates through a vapor deposition chamber.